This invention relates in general to optical fibers, and, in particular, to an optical fiber that resists attenuation caused by hydrogen and methods of making it.
The low attenuation and dispersion characteristics of optical fibers are advantageously employed to form long repeaterless links, although there is a certain amount of attenuation present in any fiber. Such attenuation ultimately requires reamplification of the light carried by the fiber. In certain circumstances it is desired to use a large percentage of the loss budget made available by the low loss (attenuation) of fiber by using long repeaterless fiber links, thereby providing very little safety factor. If after the fiber is placed in service, attenuation in the fiber significantly increases at the transmitting wavelength system operation can be interrupted.
Studies have found that attenuation of installed fibers is caused, in part, by hydrogen entering the fiber, especially the core. There are several known hydrogen induced attenuation effects: (1) interstitial hydrogen, which is directly proportional to the partial pressure of hydrogen in the ambient atmosphere, and is reversible; (2) increases in phosphorus hydroxyl absorption (1300-2000 nm) which precludes the use of P.sub.2 O.sub.5 as a dopant, except in low (less than 0.1 wt. %) concentration; (3) under high temperature-long term H.sub.2 exposure, there results a high optical absorption at short wavelengths that has an extensive tail extending through the visible and into the infrared region; (4) transient absorption that occurs when H.sub.2 first arrives in the fiber core region with most notably peaks at 1330, 1440, and 1530 nm; and permanent absorption that occurs due to Si--O--O--H--H at 1380 nm.
Others have made attempts to mitigate the hydrogen attenuation problem. See, for example, Blankenship U.S. Pat. No. 5,059,229, assigned to Corning Incorporated which describes a process for post-treating a fiber by exposure to hydrogen to reach a stable, albeit elevated attenuation level; and demonstrating no further increased attenuation when the fiber is subsequently exposed to a hydrogen containing atmosphere after being placed in service. Despite this symptomatic treatment and other efforts, the problem of hydrogen-induced attenuation persists.
One principal cause of light attenuation in optical fibers is hydroxyl groups, which produce a very strong optical absorption peak near 1380 nm. Much work has been done as evidenced by the published literature in an effort to reduce the presence of such species. It is conventionally known, for example, to dry a porous glass soot preform during consolidation in the presence of chlorine, which reacts with water present in the glass to form hydrogen chloride gas which is then simultaneously removed from the preform at high temperatures, thus reducing the concentration of hydroxyl ions in the glass.
Even granting such measures, other sources of attenuation persist. In formation of silica glasses (particularly during consolidation of the core preform and during fiber draw), peroxyl linkages (--Si--O--O--Si--) may occur, because excess oxygen becomes trapped within the glass. These peroxyl linkages can decompose, yielding reactive --Si--O--O-- sites. If hydrogen subsequently enters the glass, it can react with the --Si--O--O--species, forming Si--O--O--H--H species which absorb at 1530 nm and could therefore adversely affect operation at 1550 nm. The Si--O--O--H--H species subsequently lose a hydrogen atom and form Si--O--O--H.sub.2 which absorbs at 1380 nm. Additionally, Si--Si defects may occur. These can decompose to Si--Si-- radicals, and excess oxygen can react with them to form Si--O--O radicals. We also suspect that germanium may incorporate itself into the Si--Si defects.
We have now found that germanium dioxide can control the attenuation-increasing effects of hydrogen migration into the light-carrying regions of an optical fiber, by scavenging excess oxygen which would otherwise form reactive species, thereby preventing the reaction of such oxygen with migrating hydrogen to form hydroxyl groups. The germanium is introduced into the soot deposition flame in reactive form, e.g., germanium tetrachloride. Upon burning of the reactants including germanium tetrachloride to produce glass soot during preform lay down the germanium tetrachloride will react with oxygen to form germanium dioxide. Germanium dioxide deposited by the flame deposition process is not a stoichiometric compound because it contains fewer than 2 oxygen atoms for each germanium atom. Hence, the germanium "dioxide" can scavenge excess oxygen from the preform glass during consolidation and fiber drawing.
It is known conventionally to use germanium dioxide as a dopant in the core glass of an optical fiber preform, for the purpose of increasing the core refractive index--thereby facilitating the transmission of light through the ultimate optical fiber. During the process of consolidating the porous glass soot for the core, the chlorine used for drying has the side effect of reacting with germanium dioxide to produce germanium tetrachloride. Thus mobilized, the germanium in tetrachloride form can migrate outward from the core and redeposit as germanium dioxide.
In a small-sized preform, the migration of germanium tetrachloride outward throughout the light-carrying regions of the fiber preform due to reaction with the chlorine may be sufficient to provide enough germanium dioxide to control the excess oxygen which would otherwise be available for reaction with later-migrating hydrogen. However, this beneficial effect of the chlorine drying step depends on the diameter of the preform being dried and consolidated. The greater the preform diameter, the less effective the chlorine drying step will be in acting on available germanium dioxide in the core to spread it out into outer light carrying regions. As efficiencies of scale are achieved with drawing optical fiber of ever-increasing volume and hence increasing diameter, the need to directly address scavenging of excess oxygen through the light-carrying regions of the ultimate optical fiber accordingly becomes critical. We have found that in preforms having a diameter in excess of 105 millimeters (mm), the chlorine-induced migration of core-deposited germanium dioxide can be insufficient.